The 13-year-old boy who accidentally started the Father’s Day blaze that killed three firemen told probers in a gripping statement how he watched in fear as the flames grew and an explosion tore the building apart.

The fifth-grader, whose name is being withheld by The Post because of his age, went into hiding yesterday to avoid the media.

Investigators told The Post that after Sunday’s fire, the boy went to play basketball – but when he went home that evening, he began crying as reports of the tragedy played on the TV news. His mother said he cried in bed that night. When fire marshals came to his house Monday, his mother encouraged him to tell the truth.

The teen told the marshals he was playing behind the Long Island Supply Co. and knocked over an open can of gasoline, which spilled down a ramp and under a door to the basement – where fumes were apparently ignited by the pilot light of a water heater.

In a four-page, handwritten statement, the boy said he was playing with spray-paint cans he found behind the hardware store, a block from his house in Astoria, Queens.

“I was looking through for more cans when I knocked something over,” he wrote.

“I turned around. There was a can of gasoline. It started to pour out down the ramp. I picked it up. It leaked under the door.

“I heard some noises, and then a flame shot out from under the door. I went to get [the father of a friend who lived next door]. He came, he tried to open the door, but he couldn’t. He called the Fire Department. We stood across the street and watched the firemen work. Then we heard a loud explosion,” the statement said.

He added that his friend’s father told him, “Don’t say anything about it.”

Fire marshals unraveled the mystery when they contacted the father of the boy’s friend because he was the first to call 911.

The father told probers only that he made the call because he saw smoke. But his son, who is 15, told marshals that his 13-year-old friend had been there.

That led marshals to question the 13-year-old.

Officials said they believe the blaze that killed firefighters Harry Ford, 50, Brian Fahey, 46, and John Downing, 40, was a horrible accident, and they don’t plan to charge the boys or the father.

Marshals think the explosion was caused by fumes that collected in the basement as the fire heated cans of thinner, paint and other flammable products stored there. They no longer believe propane tanks stored in the basement played a major role in the blast.

The revelations came as wakes were held for two of the fallen Bravest.